1. Field of the Invention
The present invention relates to a method of converting gradation of a digital image and a circuit thereof and a print density controlling apparatus for a printer and a printer using this method.
2. Description of Related Art
In the case where a digital image having gradation is displayed on a display unit or printed by a printer, it is sometimes required that the number of gradation of an original image data is converted into a smaller number of gradation, for example, 256 gradation (8-bit data) are converted into 64 gradation (6-bit data). In such a case, conventionally, the method of simply converting four gradation of an original image data into one gradation has been generally used. This means that data processing has only to be performed simply in a manner that the low-order two bits of the 8-bit gradation data of the original image are neglected.
Table 1 shows a relationship of gradation conversion in this case.
TABLE 1 ______________________________________ Gradation order of 1,2, 5,6, 9,10, . . . 253,254, original data 3,4, 7,8, 11,12, 255,256 Gradation order of 1 2 3 . . . 64 converted data ______________________________________
On the other hand, in the printer of the type that thermal transfer onto print paper is performed by sublimating ink, the invention of the Japanese Patent Application Laid-Open No. 62-152,763 has been proposed as a technique for making an accurate gradation expression of an image data.
This invention adopts a configuration of controlling the pulse width of energizing to a heat generating element in response to gradation data of each picture element. This means that the relationship between the energy applied to the thermal head and the print density in monochrome is obtained for three primary colors respectively; yellow, magenta and cyan, and the applied energy for the density-scale of each color is controlled from this result.
However, in the case where a digital image data is printed by applying a simple gradation converting method as described above to a printer having such a configuration, a problem takes place that the gradation of the low-gradation part, that is, the light colored portion is visually rough. This is caused by the nature that the human eyes sense the difference in color density more sensitively for the light-color portion than for the deep colored portion (high gradation part).